The present invention relates to a method for ventilating pocket spaces in a multi-cylinder dryer of a paper machine, in particular a high-speed paper machine whose running speed is higher than 800 meters per minute, in the area of a so-called twin-wire draw in which drying cylinders in the drying group are arranged in two rows placed one above the other. Successive cylinders are placed in the upper row and in the lower row so as to form an interlocking structure. In each cylinder group there are two drying wires, an upper wire and a lower wire, by whose means the paper web is pressed against the heated faces of drying cylinders, while being guided by guide rolls placed in the gaps between the cylinders. The paper web runs as a free draw between the rows of cylinders so that the pocket spaces are formed in the area between the free draws of the web and the upper wire and its guide roll and the drying cylinder in the lower row. In a corresponding manner, an adjacent pocket spaces are formed in the area between the free draws of the web, the lower wire, its guide roll, and the drying cylinder in the upper row. The pocket spaces are ventilated by pumping air induced by the wire in the multi-cylinder dryer, wherein, at the inlet side of the guide roll, the wire pumps air into the pocket spaces through the wire and wherein the wire pumps air out of the pocket spaces at the outlet side of the guide roll of the wire.
Further, the invention relates to an apparatus for ventilating a pocket space in a multi-cylinder dryer of a paper machine in the area of a so-called twin-wire draw, in which drying cylinders in the drying group are arranged in two horizontal rows placed one above the other. Successive cylinders are placed in the upper row and in the lower row in an interlocking relationship. In each cylinder group, there are two drying wires, an upper wire and a lower wire, by whose means the paper web is pressed against the heated drying-cylinder faces, while being guided by guide rolls placed in the gaps between the cylinders. The paper web runs as free draws between the cylinder rows so that the pocket space is formed in the area between the free draws of the web and the upper wire and its guide roll and the drying cylinder of the lower row. In a corresponding manner, an adjacent pocket space is formed in the area between the free draws of the web, the lower wire, its guide roll, and the drying cylinder in the upper row.
In the multi-cylinder dryers in paper machines, in guiding the paper web, either so-called twin-wire draw and/or single-wire draw is/are employed. In a twin-wire draw, the heated drying cylinders are arranged in two horizontal rows placed one above the other. Successive cylinders are placed in the upper row and in the lower row as interlocked. In such a case, in each cylinder group, there are two drying wires, a so-called upper wire and a lower wire, by whose means the paper web is pressed against the heated faces of the drying cylinders, while being guided by the guide rolls arranged in the gaps between the cylinders. In a twin-wire draw, the web usually has free, unsupported draws as their runs between the rows of cylinders.
In a single-wire draw, only one drying wire is used in the drying cylinder group, the paper web running on support of this wire through the whole group. In the prior art, in a single-wire draw, two rows of drying cylinders placed one above the other were commonly used, but, at present, usually just one row of drying cylinders is used, whereas the other row comprises reversing suction rolls with no heating, such as the assignee's VAC-rolls.TM.. These cylinders, reversing suction rolls and drying wire are arranged in such away that the drying wire presses the web to be dried against the cylinder face, while on the reversing rolls the web is at the side of the outside curve.
A single-wire draw is usually employed in the first and second drying groups in the dryer section of a paper machine, because at the initial stage of drying the web requires constant support of the wire. As the drying makes progress, the strength of the web becomes higher and it is possible to use twin-wire groups, in which the web has free, unsupported draws between the rows of cylinders, because, in twin-wire draw, generally better drying capacity per unit of area of the web is achieved.
In the dryer sections of high-speed (velocity equals 800-1600 meters per minute) paper machines, it has been noticed that the pumpings of air induced by the wires constitute a problem for the runnability in the first twin-wire areas. Problems arising from such pumpings of air also occur in such low-speed paper machines in which the supported single-wire draw of the paper web at the beginning of the machine is short or in which wire fabrics are used that are very open in view of flows of air.
The problems have arisen from the fact that the pumping-out of air induced by the wires is, because of the same speed, usually larger than the pumping-in of air induced by the wires, in which case detrimental transverse flows are produced in the pocket spaces. Moreover, the pumpings of air induced by the wires produce interference flows detrimental for the runnability of the paper web.
As is known from the prior art, in the pocket ventilation of cylinder groups in the dryer sections of paper machines, blow-in boxes have been used, by whose means blowing has been applied through the wire. In conventional pocket ventilation, the equalizing of the pressure and the air-conditioning in the pocket space have been carried out by increasing the air flow at the inlet side of the wire. However, in these applications, a drawback has consisted of the interference with the paper web as a result of increased blowing of air, of intensification of the flowing through, and, further, of increased fluttering of the free draws of the paper web especially in high-speed paper machines, in which the pumpings of air induced by the wires have had a strong impact on the air flows and the air equilibrium in the pocket. This is why it has been necessary to reduce the permeability to air of the wires and/or it has been necessary to restrict the blowings of air in the conventional pocket ventilation. This has come especially from the fact that the problem in the pocket spaces is in itself not so much the moisture level present therein, but the problem is the pressure level present in the pocket spaces and in particular the adjustability of the pressure level, because a pocket ventilation that is in itself adequate at high speeds has been achieved by means of the pumpings of air induced by the wire.